When I open a tileset I have, Aseprite automatically imports the colors in the image. But for some reason I cannot use any colors other than what is there on the pallet!! This does not happen with any other images I tried, only the certain tile sets I have. Can someone tell me what is going on?!

This can depend on the color mode of the png. PNG files can be RGB or Indexed. If the sprite is indexed you can use only colors in the palette, if it’s RGB, you can use any color in the image (and the palette is just for reference). You can check/change the color mode from Sprite > Color Mode menus.